I. Field of the Invention
The present invention relates to a method and an apparatus for coding/decoding telewriting signals such as handwriting signals representing handwritten characters and graphics for transmission and display in a telewriting system.
II. Brief Description of the Prior Art
A communication system for simultaneously transmitting voice and handwritten information through an ordinary telephone channel is called a telewriting system. The bit rate allowed in transmitting handwriting signals must be maintained at about 300 bits/sec or less unless major degradation in voice quality is permissible. A coding method which enables handwriting signal transmission at a rate of 300 bits/sec or less is therefore essential to the telewriting system. The real-time handwriting signals to be transmitted must be sequentially processed by a coding algorithm. Furthermore, a simple, compact encoder must be used since it is built into a telewriting terminal.
Various coding techniques to store line drawing data have been proposed in the fields of picture processing, picture recognition, and computer graphics. Two coding methods suitable for the telewriting system have been conventionally known: (1) differential chain encoding (DCE) and (2) modified DPCM (MDPCM).
(1) DCE Method
In the DCE method, a change in direction of a drawn line is coded. A window comprising a 5.times.7 pel matrix is formed such that a pen position matches with the center of the matrix. The movement of the pen across any one of the peripheral 20 pels in the window is detected. An exponent corresponding to the direction of movement of the pen is determined by the pel position crossed by the pen. In a system wherein such exponents are sequentially obtained, a difference between two adjacent exponents is obtained and coded to a variable-length code, thereby encoding the change in direction of the drawn line.
The DCE method has a considerably high coding efficiency when the handwriting speed is slow. However, when the handwriting speed is increased, the coding efficiency is abruptly decreased. Furthermore, since part of the picture information is omitted by the window, the picture resolution is degraded accordingly.
(2) MDPCM Method
Unlike the DPCM method wherein coordinate differences d.sub.xi (=x.sub.i -x.sub.i-1) and d.sub.yi (=y.sub.i -y.sub.i-1) of pen positions sampled at predetermined time intervals are coded, the MDPCM encodes the differences d.sub.xi and d.sub.yi when the absolute values of the differences d.sub.xi and d.sub.yi are equal to or smaller than a predetermined value d.sub.max (i.e., 0&lt;.vertline.d.sub.xi .vertline..ltoreq.d.sub.max and 0&lt;.vertline.d.sub.yi .vertline..ltoreq.d.sub.max), and encodes an absolute coordinate point (x, y) when the absolute values of the differences d.sub.xi and d.sub.yi are greater than the predetermined value d.sub.max (i.e., .vertline.d.sub.xi .vertline.&gt;d.sub.max or .vertline.d.sub.yi .vertline.&gt;d.sub.max). The MDPCM method has a high coding efficiency when the handwriting speed is low. However, when the handwriting speed is increased, the coding efficiency is decreased with an increase in the amount of data to be processed. Furthermore, a high-resolution image cannot be effectively encoded at a high coding efficiency.
As may be apparent from the above description, depending on the types of telewriting signals, the conventional coding method may not satisfy the condition wherein a voice signal and a handwriting signal are simultaneously transmitted at a bit rate of 300 bits/sec or less. Therefore, the bit rate must be further decreased.